Several different types of data cables are in use today. Some data cables utilize optical fibers to transmit light signals, while others use conductors to convey electrical data signals. To minimize potential incompatibility between data cables of the same general type, standards have been established. For conductive data cables, one such standard is known as TIA/EIA-568-B for eight-conductor, 100-ohm, balanced, twisted-pair cabling, such as category 5e conductive data cables. The most identifiable feature of category 5e data cables are their pin/pair assignments. The pin/pair assignment of category 5e cables is often referred to as “eight position eight conductors,” (“8P8C”) or sometimes referred to as “RJ45.” Category 5e conductive data cables are often used in commercial settings where a spectrum of at least 100 MHz is required for data transmission. Typical applications include 10 base T, 100 base TX, token ring, 1000 base T gigabit Ethernet, 155 Mbps ATM, or 622 Mbps ATM.
Depending on the location, to effectively convey data signals from one location to another, a conductive data cable must minimize or prevent moisture inside the data cable since high moisture levels can degrade conductivity and result in loss of data or data distortion. Depending on the construction of the particular data cable, the introduction of moisture can result in a short circuit, a decrease in the data cable's impedance, an increase in signal attenuation, or in the complete failure of the data cable.
Moisture can penetrate to the interior of the data cable in several different ways. Water may enter through a failure in a data cable's jacket. Water may also enter through a cable end, where a cable connector is attached. Mechanical impacts, electrical arcs, or lightning may breach the jacket that protects the data cable or the joint where one data cable joins another. Water may then flow through the breach towards the core of the data cable and longitudinally along the length of the data cable. Also, changes in ambient conditions may lead to differences in water vapor pressure between the interior and the exterior of the data cable. The difference in vapor pressure may then cause moisture to diffuse into the interior of the data cable. Eventually, there may be an undesirable level of moisture inside the cable.
Since the data cable's ability to resist penetration by moisture may be a crucial characteristic in certain applications, the data cable must be tested and meet certain performance specifications to ensure that the presence of water will not significantly affect the data cable. Several different performance specifications pertain to waterproof data cables. The particular specification used depends on the proposed application and use. One such specification is MIL-DTL-24643/59, which is set by Naval Sea Systems Command. It prescribes the water blocking requirements for a conductive data cable to be used on a Navy ship. To meet the requirements of MIL-DTL-24643/59, an open end of the data cable is subjected to a predetermined water pressure for a predetermined amount of time. Data cables that allow limited water migration to a specified length when subjected to the test are deemed “waterproof.”
Various methods have been used to block water. One method of protecting data cables against water penetration is to provide a layer of plastic or polymeric material. In a cable insulated by a polymeric material, water can travel by capillary action along the cable interstices, causing problems in conductivity. In most environments, it is desirable, if not essential, that the cable be more watertight than can be achieved with polymeric material alone. Some data cables may include a metal/plastic laminate foil beneath the outer protective jacket of the data cable. The metal/plastic laminate foil may become bonded to the polymeric material, normally when the polymer is extruded. However, it is difficult to design a jacket in which the laminate foil remains intact when the data cable is subjected to impact or bending during or after installation as the laminate tends to be driven into gaps between conductors lying underneath the laminate and cracks quickly along the resulting crease lines.
Another method of protecting a data cable against water penetration is to use water swellable materials. However, when water swellable materials are exposed to high humidity over a period of time, they expand by as much as three times their original volume. Associated dielectric properties of water swellable materials, such as dissipation factor and dielectric constant, change as water swellable materials absorb moisture. The water swellable materials are generally in close proximity to the insulated conductors of the data cable. Thus, changes in the dielectric properties of the water swellable materials affect the dielectric properties of conductive data cables, and changes in the dielectric properties of conductive data cables affect their data transmission capabilities. Therefore, when the dielectric properties of the water swellable materials change, the change affects the data transmission capabilities of conductive data cables.
Filler materials are also commonly used in conductive data cables to prevent water penetration by capillary action along cable interstices. Filler materials are commonly synthetic polymers, petroleum based greases, oils, or silicone flooding compounds. Filler materials may be coated on components of the conductive data cable to prevent longitudinal movement of moisture. In addition, the interstices within the cable may be filled with the filler material to minimize water entry and migration. However, applying filler material in order to block water necessitates additional handling and processing steps in the manufacturing of the cable. The additional steps increase manufacturing time. Further, the addition of filler material significantly increases the weight of the electrical cable. Finally, moisture blocking filler material is typically difficult to remove during termination which significantly increases termination time.
Thus, there is a need in the art for an invention to provide better protection of data cables against water penetration. Particular need remains for water blocking protection that does not change the transmission properties of the data cable. Also, the water blocking protection must be easily removed from the conductors during termination. Furthermore, the water blocking protection must allow the cable to meet the requirements of MIL-DTL-24643/59. Lastly, the water blocking protection must not cause failure of the propagation delay and delay skew for cables with multiple pairs of conductors.